The presence of large deposits of oil shale in the Rocky Mountain region of the United States has given rise to extensive efforts to develop methods of recovering shale oil from kerogen in the oil shale deposits. It should be noted that the term "oil shale" as used in the industry is in fact a misnomer; it is neither shale nor does it contain oil. It is a sedimentary formation comprising marlstone deposit interspersed with layers containing an organic polymer called "kerogen", which upon heating decomposes to produce carbonaceous liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein, and the liquid carbonaceous product is called "shale oil".
A number of methods have been developed for processing the oil shale which involve either first mining the kerogen bearing shale and processing the shale above ground, or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact since the spent shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes.
The recovery of liquid and gaseous products from oil shale deposits has been described in several patents, one of which is U.S. Pat. No. 3,661,423, issued May 9, 1972 to Donald E. Garrett, assigned to the assignee of this application, and incorporated herein by reference. This patent describes in situ recovery of liquid and gaseous carbonaceous materials from a subterranean formation containing oil shale by mining out a portion of the subterranean formation and then fragmenting and expanding a portion of the remaining formation to form a stationary permeable mass of explosively expanded and fragmented formation particles containing oil shale within the formation, referred to herein as an in situ oil shale retort. Hot retorting gases are passed through the in situ oil shale retort to convert kerogen contained in the oil shale to liquid and gaseous products.
One method of supplying hot retorting gases used for converting kerogen contained in the oil shale, as described in U.S. Pat. No. 3,661,423, includes establishment of a combustion zone in the retort and introduction of an oxygen supplying gaseous feed mixture as a combustion zone feed downwardly into the combustion zone to advance the combustion zone downwardly through the retort. In the combustion zone oxygen in the gaseous feed mixture is depleted by reaction with hot carbonaceous materials to produce heat and combustion gas. By the continued introduction of the oxygen supplying gaseous feed mixture downwardly into the combustion zone, the combustion zone is advanced downwardly through the retort.
The combustion gas and the portion of the gaseous feed mixture which does not take part in the combustion process passes through the retort on the advancing side of the combustion zone to heat the oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called retorting, in the oil shale to gaseous and liquid products and a residue product of solid carbonaceous material.
The liquid products and gaseous products are cooled by the cooler oil shale fragments in the retort on the advancing side of the retorting zone. The liquid carbonaceous products, together with water produced in or added to the retort, are collected at the bottom of the retort and withdrawn to the surface through an access tunnel, drift or shaft. An off gas containing combustion gas generated in the combustion zone, product gas produced in the retorting zone, gas from carbonate decomposition, and gaseous feed mixture which does not take part in the combustion process is also collected at the bottom of the retort and withdrawn to the surface.
The off gas, which contains nitrogen, hydrogen, carbon monoxide, carbon dioxide, methane, water vapor, hydrocarbons, and hydrogen sulfide, and is essentially free of free oxygen, can be used as a fuel or otherwise disposed of, but should first be purged of the hydrogen sulfide, which is a pollutant. The hydrogen sulfide, which can be present in the off gas at concentrations in the range of 1500 to 3000 parts per million (ppm) by volume, is generated from naturally occurring sulfur compounds in oil shale during the heating and combustion in the in situ oil shale retort.
Hydrogen sulfide is an extremely toxic gas with a toxicity greater than that of hydrogen cyanide. It also possesses a powerful, objectionable odor with a threshold for human smell of about 0.0003 ppm. For these reasons emission standards for hydrogen sulfide have been established in many states, including states having oil shale deposits. Thus, various processes for the removal of hydrogen sulfide from gases such as off gas from oil shale retorting have been devised. These processes generally involve absorption of hydrogen sulfide into a liquid such as alkanolamine or high temperature liquid carbonate solutions, adsorption of hydrogen sulfide onto a solid such as iron oxide pellets, and catalytic or noncatalytic oxidation of hydrogen sulfide to sulfur and/or sulfur dioxide such as in the Claus process.
A problem with absorption and adsorption processes is that the agent used for absorbing or adsorbing must, after use, be chemically regenerated or disposed of and replaced. Either of these alternatives can be expensive. A problem with noncatalytic oxidation is that high temperatures are required which may result in oxidation of the hydrocarbon and carbon monoxide constituents of the off gas, thereby substantially reducing the heating value of the off gas. Even with catalytic oxidation, temperatures can be so high that substantial reduction in the heating value of the off gas occurs. Another problem with catalytic oxidation is that the catalyst eventually becomes poisoned, thereby exhibiting reduced activity, and must then either be chemically regenerated or disposed of and replaced.
Thus, there is a need for an economical method for decreasing the hydrogen sulfide concentration of a gas stream, such as the off gas from an in situ oil shale retort, where the method does not substantially reduce the heating value of the gas stream.